How to set up the internal trimmers !?

[ODwan]

Hello!

I recently finished my Hybrid A clone, done some tweaking by the suggestions on this forum (special thanks are the great dogears due for his free sharing of his superb knowledge and ear!). Great sounding amp!

Now I am about to set up the internal trimmers and would like to ask if any of you knowledgable guys has special recommendations, especially concerning the PI balance trimmmer.

Thanks in advance,
Timo

[Bob-I]

The OD entrance trimmer I found sounds best set to a total resistance from Input to the grid of 235K. Measure from the relay to the grid.

The PI is trickier. You need to balance the AC voltage on both sides of the PI output. For this you'll need a scope and a sine wave generator..... yea... I don't bother either, I just set in the middle.

[jaysg]

The OD entrance trimmer I found sounds best set to a total resistance from Input to the grid of 235K. Measure from the relay to the grid.

uh...k, there's the relay, "100k or 220k" 100kB OD Trim, and a 68k grid resistor on OD1. Do you mean 235k - 68k = 167k? That would mean it's the 100k, not the 220k drop resistor I guess. The pot is 100k, and is set at the 1/3 point above ground, assuming an accurate 100k linear pot?

[Bob-I]

uh...k, there's the relay, "100k or 220k"[/quote]

This is one of the issues with these "best guess" schematics. I use a 150K in this position. Scott suggested it and to my ears it sounds best.

100kB OD Trim, and a 68k grid resistor on OD1. Do you mean 235k - 68k = 167k? That would mean it's the 100k, not the 220k drop resistor I guess. The pot is 100k, and is set at the 1/3 point above ground, assuming an accurate 100k linear pot?

I was just reading from my notes when I posted that... let's see...

150K+100K pot +68K. That leaves about 17K on the pot, almost full up.

Then of course it depends on the tubes and the plate/cathode resistors.

[dogears]

68k is in addition.... I use anywhere from 180k to 220k in front of the 100k trimmer.... The 235k value has nothing to do at all with the 68k. It was only a guideline if using something like a 180k. If you use a 200-220k you'd obviously have more than 235k. Maybe 250k.....

The OD entrance trimmer I found sounds best set to a total resistance from Input to the grid of 235K. Measure from the relay to the grid.

uh...k, there's the relay, "100k or 220k" 100kB OD Trim, and a 68k grid resistor on OD1. Do you mean 235k - 68k = 167k? That would mean it's the 100k, not the 220k drop resistor I guess. The pot is 100k, and is set at the 1/3 point above ground, assuming an accurate 100k linear pot?

[jaysg]

k, that gives me something to experiment with. I've got 100k + 100k pot and I'm running it pretty high which you guys would say is hitting OD1 too hard...hmm.

[Bob-I]

k, that gives me something to experiment with. I've got 100k + 100k pot and I'm running it pretty high which you guys would say is hitting OD1 too hard...hmm.

Yes, sounds too high. Go to a 150-170K and you'll be in better shape.

[ODwan]

I also used 100k in front of the 100k trimmer. Today I experimented with 220k and 180k. 220k seemed to dull the hi end just a tad too much and yielded too low a gain for me. 180k has just the right sweetening in the highs and still enough gain to my ears. It stays right in there!

As to the PI trimmer I hooked up my old oszi and tapped the signal off at the grid leak resistors. In order to have equally large waves on each side I had to krank the trimmer to the non inverting side. Thus I now have 100k on the inverting and 120k on the noninverting. Btw, I could not discern a change in tone. I guess it does not matter that much until one overdrives the PA.
As a side effect I discovered a tiny oscillation of about 50kHz in the PA. Changing the MV does nothing to it, so I gues it's local. Need to put a 47pF across the PI plates and see what it does.
Thanks so far,

Timo